This invention relates generally to fluidized beds in which solids and fluids flow in a countercurrent relationship, and, more particularly, to the use of internal structures to facilitate contact between the solids and fluids in the fluidized bed.
Fluidized beds are frequently used in petroleum, chemical, combustion, and other types of processes to promote vigorous mixing and intimate contact of fluid streams and solid particles within a vessel. This intimate contacting can be used to achieve efficient heat transfer, mass transfer and/or chemical reaction between the fluid streams, solid particles, and/or fluids coated on or entrained with the solid particles. Fluidized beds are typically generated by passing the fluid stream, typically a vapor stream, upwardly through a bed of small solid particles at a flow rate sufficient to suspend the particles and cause a turbulent mixing of the solid particles. The lower boundary of the fluidized bed is formed at, or just below, the level of the fluid stream inlet. The upper boundary varies in relation to the velocity of the fluid stream and is formed at the level where the fluid disengages from the particles. The velocity of the fluid flow is maintained above that which will cause suspension of the solid particles and below that which will cause the particles to be carried out of the vessel or above the desired upper boundary level.
In some types of fluidized beds, the solid particles remain suspended in the fluidized bed and there is no net downward flow of the solid particles. In other types of fluidized beds, the solid particles are continually added at the top and removed from the bottom of the fluidized bed so there is a resulting downward flow of solid particles countercurrent to the upwardly flowing fluid. In both types of fluidized beds, it is generally desirable to reduce channeling of the fluid through the solid particles and the formation of stagnant zones of fluid or solid particles in the fluidized bed. It may also be desirable, particularly in the case of countercurrent fluidized beds, to reduce recirculation or backmixing of the solid particles and fluid within the fluidized bed because of the detrimental effect backmixing may have on the efficiency of the particular process occurring within the fluidized bed.
An example of a fluidized bed involving countercurrent flow of fluid streams and solid particles is found in certain types of strippers and regenerators used in fluid catalytic cracking or FCC systems. In such FCC systems, intermediate and high-boiling point hydrocarbons are atomized and brought into contact at high temperature with fluidized catalyst particles in a reactor whereby the hydrocarbons are cracked to produce lower boiling point reaction products such as gasoline. The reaction products and catalyst particles are then separated, such as in a cyclone, and each proceeds separately for further processing. The catalyst particles are typically removed from the reactor in a continuous fashion and subjected to further processing, first in a catalyst stripper to remove volatile hydrocarbons and then in a regenerator to remove nonvolatile carbonaceous material, called coke, which is deposited on the catalyst particles during the reaction process and reduces the effectiveness of the catalyst. In the catalyst stripper, entrained, interstitial and adsorbed volatile hydrocarbons are removed from the catalyst in a fluidized bed by countercurrently contacting the catalyst with a flowing gas stream, such as water vapor, in a process referred to as stripping. Removal of these residual hydrocarbons from the catalyst is desirable because the hydrocarbons may be recovered and returned to the process as a reaction product, rather than being conveyed with the catalyst particles to the regenerator where they would be combusted, thereby causing an increase in air demand to the regenerator. Combustion of the residual hydrocarbons in the regenerator may also contribute to degradation of the catalyst by subjecting the catalyst to elevated temperatures. The catalyst particles leave the stripper and are then directed to a regenerator where the coke deposits and any residual hydrocarbons are burned by passing the catalyst particles through a fluidized bed countercurrent to an oxidation gas, typically air, in a process referred to as regeneration. The regenerated catalyst particles are then returned to the reactor for further catalytic cracking of hydrocarbons. In these fluidized beds found in the FCC stripper and regenerator, it would be desirable for all of the catalyst particles and the fluid streams to pass through the fluidized beds in a fully countercurrent fashion without channeling and backmixing and with all catalyst particles and gas streams passing through the fluidized beds within defined time intervals, a condition known as plug flow, so that better and more predictable process efficiencies can be obtained.
It has been reported that devices, such as random packings, which have been used to approach the condition of plug flow in countercurrently flowing gas and liquid systems, do not necessarily work well in gas and solid particle systems because the solid particles can lodge in unaerated stagnant zones within the packing. It has also been reported that, through trial and error testing, some grid-type packings, such as chevron or disc and donut elements, have proven relatively effective in retarding the rate of top to bottom mixing of solids in well-fluidized beds. These grid-type packings, however, can reduce the quantity of fluids and solids that can pass through the fluidized bed because the packings force the fluids and solids to flow through constricted flow paths. In addition to reducing the flow capacity, the packings often have poor "turndown" performance because they offer acceptable processing efficiency only within a limited range of gas flow rates. Still further, these packings may permit large gas bubbles to form in the fluidized bed with several undesired consequences, including reducing the contacting efficiency between vapor and solids, increasing backmixing of solids by upward displacement of solid particles by the gas bubbles, and increasing entrainment of the solids into the dilute phase above the fluidized bed as a result of the large gas bubbles bursting upwards through the fluidized bed. As a result, a need has developed for a packing-type element that restricts less of the cross-sectional flow area of the fluidized bed, performs well across a wider range of gas flow rates, and reduces the formation of large gas bubbles within the fluidized bed.
Static mixing elements consisting of rigid forms are conventionally used for purposes such as to achieve thorough mixing, mass transfer, heat transfer, or chemical reaction in streams of flowable substances flowing co-currently through a pipe, vessel or other conduit. These elements can take many forms but typically utilize stationary deflectors that split, shear and then recombine the fluid streams or fluids and solids until a generally homogenous stream exists. Static mixers are typically of specialized design for specific use applications, such as those involving either liquid-liquid, liquid-solid, or gas-solid co-current flow, because good performance in one type of application doesn't necessarily indicate that the static mixer will perform well or even acceptable in other applications.
It has been suggested that one type of static mixing element, commonly known as an SMV element, can be used in liquid-solid fluidized beds to achieve higher solids concentration under certain liquid flow conditions. The SMV element comprises a bundle of corrugated sheets that are positioned so that corrugations of adjacent sheets are in contact with and extend at angles to each other, thereby forming liquid and solids flow paths along the peaks and valleys of the corrugations. The effect of the SMV element on backmixing of the solids, and the suitability of the element for use in gas-solid rather than liquid-solid fluidized beds, were not reported.
It has also been suggested in U.S. Pat. No. 5,716,585 that corrugated sheets of packing, such as modified SMV elements, can be used to facilitate stripping of solids in gas-solid fluidized beds. In that patent, the use of the corrugated sheets of packing in stripping units for spent FCC catalysts is specifically disclosed. The impermeable nature of the corrugated sheets, however, blocks passage of gases and solids through the sheets and may serve as an impediment to the desired exchange between stripping gas and hydrocarbons associated with the catalyst particles.
Another type of static mixing element is disclosed in U.S. Pat. No. 4,220,416 to Brauner et al. The element disclosed in that patent comprises pairs of planar portions arranged in spaced apart relationship in two perpendicular planes and joined together along a connecting spine, with a plurality of paired planar portions typically being placed end to end within a pipe or other conduit. Each planar portion comprises at least one web, and normally two or more webs that are spaced apart to provide open slots through which substances may flow for mixing. Although also used for other applications, these types of elements have proven particularly useful in mixing highly viscous polymer compounds flowing in co-current laminar flow. To date, there have been no reports suggesting the suitability of these elements for use in fluidized beds.